1. Field of the Invention
The present invention relates generally to the field of interferometry and more particularly, to a variable shear A.C. interferometer based on a rotating grating.
The invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the Department of the Air Force.
2. Description of the Prior Art
Interferometers are commonly used to measure wavefronts, and, in particular, are employed in a wide variety of applications to measure the departure of a wavefront from a true sphere (or from a plane wave in the case of collimated light). Such departures of a wavefront from its true shape typically occur as a result of imperfections in the optical path along which the wavefront travels. For example, the imperfections could be caused by a lens or some other optical system through which the light travels, and, in this situation, the interferometer could be used as an instrument to test the lens or other optical system.
In another application, the imperfections could be as a result of the effects of a turbulent atmosphere through which the light travels, and the interferometer can be employed to measure this distorted wavefront in a system to compensate for the effects of the atmosphere to permit more accurate imaging of distant objects such as a star or the like.
In general, interferometers operate by converting phase differences in the wavefront into intensity differences that can be monitored and read with a detector, and a variety of interferometers are known in the prior art. In U.S. Pat. No. 3,829,219 of James C. Wyant entitled "Shearing Interferometer," a prior art A.C. grating interferometer based on double frequency phase gratings is described. In this system, known as the Wyant Interferometer, a wavefront being observed is converged to a focal point, and two diffraction gratings having slightly different line spacings are placed near the focal point of the wavefront. The diffraction gratings produce two first diffraction orders at two slightly different angles which result in a shearing interferogram in the region of overlap. This interferogram yields wavefront information in one direction, and by shearing the wavefront in two orthogonal directions as by providing two additional diffraction gratings, orthogonally oriented relative to the first two diffraction gratings, complete wavefront information in two directions can be obtained.
The interferometer described in the above patent, as well as most other prior art shearing interferometers suffer from several inadequacies. For one thing, they are essentially designed for use with coherent light and do not operate effectively, if at all, with white light. Also, their optical efficiency is usually quite limited because in such systems the diffraction orders are spatially separated and each are sheared with the result that riot much light gets through to each of the various orders.